1. Field of the Invention
This invention relates generally to multi-focal lenses, and more particularly to nonprogressive bifocal lenses that eliminate image jump, distortion, and loss of image as one moves their eyes from the distance segment to the reading segment. The instant invention also provides a safer bifocal lens in that it has a smaller reading segment that does not obstruct ones peripheral vision and vision while climbing stairs and when walking on the streets and sidewalks. The lens provided by the invention offers a convenient, useful, non-obstructive and safe alternative for active people over forty years of age.
2. Description of the Background Art
Multi-focal lenses generally comprise bifocals and trifocals. Bifocals include two lenses having different curvatures to offer the convenience of distance and reading vision in the same pair of eyeglasses. Bifocals compensate for loss of accommodation, i.e. presbyopia. Trifocals typically provide a third lens adjacent the second for improving vision in the near viewing range, but outside the reading range. The most popular bifocals today are the flat top lens and progressive lens. Despite the convenience offered by flat tops, progressive and other multi-focal lenses, there have been compromising tradeoffs, such as image jump, loss of image, distortion, loss of peripheral vision and obstruction or distortion when looking down. The loss or obstruction of peripheral and downward vision interferes with the side rear view mirrors when driving and the ability to see clearly when climbing stairs, walking in the streets and entering and exiting buses and street cars. This is because conventional distance and reading segments are separated by visible lines and/or uncontrolled random blending, resulting in image jump, image loss, and loss of continuity. Conventional reading segments are also larger than necessary. Objects become hidden behind the dividing line and outer portions of the reading segment creating confusion and longer vision adaptation periods. Accordingly, these tradeoffs are a result of inherent design flaws which the industry has unsuccessfully tried to correct.
The evolution of the multi-focal lens began with the bifocal, invented by Ben Franklin. The original bifocal, also known as the executive bifocal, comprises an upper lens and lower lens divided by an unsightly, thick center line, as seen in FIG. 1a. In addition to being unattractive, objects disappear behind the dividing line when the eye moves between the top and bottom vision areas. In the 1890s, the round bifocal, shown in FIG. 1b, was introduced as a cosmetic improvement over the executive, but it introduced excessive image jump and provided a smaller reading area.
With reference to FIG. 1c, the most popular bifocal lens is the flat top 1. The flat top bifocal was introduced in the 1920s and the FT-28 has become the most widely used bifocal eyeglass. The FT-28 stands for a flat top lens having a reading segment with a 28 mm width. Flat tops are simply round bifocals with the upper portion cut away or removed, bringing the optical center of the reading segment closer to the top line, thereby reducing image jump by 50 percent. Nonetheless, 50 percent of the image jump still remains, continuing to cause patient adaptation problems and visual disturbance. Like the executive, objects still become lost when entering the segment area through the top of the line.
Referring to FIGS. 1c, 9 and 10, the flat bifocal lens obstructs distance vision when driving and climbing stairs because of its size and construction. The reading segment size impairs downward distance vision and the corners 3 obstruct peripheral vision, thus affecting a driver's ability to use the side rear view mirrors. The reading segment also has an upper edge and dividing line 4 which causes image jump and distortion when moving the eyes between the distance area and reading segment. In addition, the reading segment 2 extends low such that it obstructs vision when looking down to climb stairs, steps, etc.
Progressive lenses 5 and blended lenses, as seen in FIGS. 2a and 2b, respectively, were introduced to remove visible segment lines. Progressive lenses provide a continuous range of focal powers by using large numbers of radii in an attempt to give a gradual progression in vertically increasing power. The cosmetic benefits offered by progressive lenses is offset by peripheral astigmatism, distortion, aberrations, and loss of peripheral vision. The problem with progressive lenses is that each progressively shorter radius leaves an area of uncorrectable distortion as it departs from the connecting point, which is the vertical centerline of the segment. All progressive lenses include blended areas 6 which extend all the way to the edges of the lens, accounting for substantial loss of peripheral vision, as shown in FIG. 2b. Although the extended blended areas 6 are intended to minimize concentrated distortion, they still increase the range of distortion. The distortion zones found in most progressive lenses affect approximately 55 to 60 percent of the lens. Although the aesthetically unacceptable lines are not visible in progressive lenses, the wearer is plagued with excessive distortion. Another deficiency of progressive lenses is that they have narrow, long corridors 7 of power increase which are difficult for opticians to correctly position, especially for reading. It is also difficult for the user to aim the lens toward the reading area. Moreover, progressive lenses interfere with peripheral vision as shown by the circle 3.
In accordance with the above discussion, some examples of blended and progressive multi-focal lenses may be found in U.S. Pat. No. 4,952,048 issued to Frieder et al., 5,123,725 issued to Winthrop, 5,106,180 issued to Marie et al., and 4,955,712 issued to Barth et al. U.S. Pat. No. 5,351,100 issued to Schwenzfeier et al. discloses a glass multi-focal lens with a polarizing element to combine the features of multi-focal lenses with light protecting elements. Composite plastic lenses having thin lenses heat bonded together offer another type of multi-focal lens as discussed in U.S. Pat. No. 4,969,729 issued to Merle. Some of the problems associated with image jump and separation of image as one moves their eyes from distance to reading are addressed in U.S. Pat. No. 4,869,588 ('588 Patent) issued to Michael Walach, the inventor herein, and Frieder. The lens in the '588 Patent, shown in FIG. 2a, still obstructs vision when walking and driving, obstructs peripheral vision and has aggravating distortions around the reading area, amounting to 25% of the eyeglass area.
Despite the number of different types of multi-focal lenses, none has sufficiently addressed nor solved the problems noted herein. Accordingly, there exists a need for a multi-focal lens that is safe for walking, driving, climbing stairs and other activities. The needed lens does not have dividing lines, progressive corridors and blend zones and eliminates image jump, loss of image and distortion in a nonprogressive type of lens. The instant invention addresses these needs.